Practical Reason The subject of most of the Transcendental Dialectic is the speculative use of reason. In its speculative use, reason attempts to bring what is under the the scope of its ideas. In its practical use, on the other hand, reason is concerned with what ought to be , which has no necessary connection to what is. Practical reason is concerned with the question: what ought I to do? In the Preface to the second edition, Kant stated that his criticism of metaphysics has "a positive and very important use," (Bxxv) in that it removes obstacles threatening to destroy practical reason. Certain metaphysical doctrines, namely, "materialism, fatalism, [and] atheism" (Bxxxiv) must be curbed in the interests of practical reason, and the critique of metaphysics serves to police the realm of the practical against incursion from metaphysics. "The dogmatism of metaphysics, that is, the preconception that it is possible to make headway in metaphysics without a previous criticism of pure reason, is the source of all that

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